Carpet and upholstery cleaning devices typically direct a stream of water-based cleaning solution directly onto the material to be cleaned. Along with the solution utilized and concurrent vacuuming, the volume and temperature of the solution significantly affect the ability of the cleaning device to remove dirt from the material. Cleaning devices typically use water heaters which have relatively low volume output in order to maintain the proper temperature.
In general, most prior art electric water heaters have a chamber wherein water is circulated around a heating element which heats the water to a desired temperature. The volume of the chamber is usually relatively large compared to the volume required by the heating element. The temperature of the water is monitored by a thermostat which is located within the chamber at some distance from the heating element. The thermostat must also be sealed and insulated and can be rather expensive. Internal thermostats present problems under high pressure operating conditions in the range of 1000 psi.